Chemical composition and process for aluminum etching



United States Patent O CHEMICAL COMPOSITION AND PROCESS FOR ALUMINUM ETCHING Robert Houston Elliott, Jr., Oreland, Pa, assignor to Pennsalt Chemicals Corporation, a corporation of Pennsylvania N Drawing. Filed Nov. z, 1955, Ser. No. 544,583

5 Claims. (Cl. 41-42) This invention relates to the etching of aluminum and aluminum base alloys and more particularly to the rapid etching of aluminum and aluminum base alloys and compositions for obtaining such rapid etch.

The term etching as used herein, both in the specification and claims, includes not only light surface etching, but chemical dissolution of substantial amounts of the metal treated as occurs when forming metal objects through controlled selective chemical dissolution, a process frequently referred to as chem milling.

It is frequently desirable in the fabrication of construction parts from aluminum, for example in the fabrication of airplane parts, to produce parts which are relatively light in weight and which still retain relatively high mechanical strength. In order to reduce the weight of such parts without impairing their strength, it is the practice to remove as much of the metal as possible from the portions 'of the part which do not contribute to its mechanical strength. It is sometimes extremely difiicult, if not impossible, to accomplish this by the conventional methods of mechanical milling or machining for the reasons that the parts may be highly complex in shape. Also, by mechanical removal of the excess metal the strength of the part may be adversely affected.

For these reasons, the practice has recently developed of removing this excess metal by chemical means,

It is known that construction parts made from aluminium and aluminum base alloys can be subjected to an alkaline etching bath, under controlled conditions of concentration and temperature, whereby metal, not protected, is dissolved. The portions of the part from which it is not desired to remove metal are protected by some suitable masking material which usually consists of a coating of chlorinated neoprene, polyvinyl chloride or latex-base film 'or other alkali-resistant coating. In addition, delicately fretted or perforated articles are frequently manufactured by mechanically working. two metals, only one of which is soluble in a particular etching bath, into the desired shapefland then removing the soluble metal in the etching bath. By these methods, highly complex shapes are more easily formed.

'It has now been discovered that the etching rate of alkaline etching baths for aluminum and .aluminum base alloys and particularly aluminum alloys containing about 0.1% or more copper, can be substantially increased by the addition of small amounts of a salt of divalent cobalt. The rate of attack of an alkaline etching bath on aluminum is dependent to some degree on such factors as the concentration of the alkali and the temperature at which the bath is operated. The cobaltous ion in no way counteracts the increase in rate obtained by increasing alkali concentration and temperature and the additive may, if desired, be used in conjunction with higher alkali concentrations and temperatures to obtain even faster etching rates.

The cobaltous ion maybe added to the etching solution in the form of any salt that is wholly or partially soluble in water. The cobaltous ion is relatively more a soluble complex; in dilute alkaline solutions of, for example, less than 5% castic soda, the cobalt precipitates even in quite low concentrations. An additive which forms an insoluble compound on addition to alkaline solutions might not be expected to produce an eifect of any kind in an alkaline etching bath, but surprisingly, provided the ion is introduced into the bath in the form of a wholly or partially water-soluble salt, it produces an accelerating effect. Although the exact mechanism is not known, it may be that a minute but sufiicient amount of the ion is present in the form of .a colloid or complex of some description to provide the desired eflfect. Examples of suitable cobaltous salts are cobaltous chloride, cobaltous chloride dihyd'rate, cobaltous acetate, cobaltous benzoate, cobaltous bromide, cobaltous nitrate, cobaltous sulfate, etc. a

Most aluminum etching baths are made up using caustic soda, for reasons of economy, although other etching alkalies can be'used. Similarly, although caustic soda is the preferred alkali for the practice of thepi'esent invention, other alkalies such as potassium hydroxide, lithium hydroxide or trisodium phosphate may be used. In conventional practice, the' caustic soda concentration of aluminum etching baths is generally in the range of from 1% to not more than about 20%, most chem'milling being done within the range of 8 to 15% since the lower concentrations generally do not contain suflicientf caustic to remove theilarge amount of metal required and at the higher concentrations the baths tend to foam and spatter excessively. Use of the cobaltous ion, however, is not limited to these concentrations, the presence of these metal ions further accelerating the very rapid etching baths of applicants'co-pending application Serial No. 544,584, filed November 2, 1955, now abandoned, in which highly concentrated alkaline baths in the order of 30 to 55% caustic soda are used.

Several additives'are known to reduce or prevent the scale formation that normally accompanies alkaline aluminum etching. Examples of additives suitable for this purpose are those described in US. Patent No. 2,584,017 and applicants co-pending application Serial No. 499,- 502 filed March 31, 1955, now US. Patent No. 2,872, 301, the preferred scale inhibitors generally being the glucamines, sorbitol and gluconate. inhibitors are used they are generally present in amounts of l to 5%, based on the amount of alkali. It is preferred to include one or more antiscalantsin the fast etching compositions and process of the present invention, although such antiscalant is not necessary for successful acceleration butis included primarily to prevent scale formation. When large amounts of aluminum are.

to be removed, as is generally the case where very fast etching baths are employed, it is advantageous to precontaining the cobalt accelerant.

Very small concentrations of the cobaltous ion, on the order of 0.2 to 2.5% ion based on the weight of alkali, 1 will provide increased acceleration for alkaline etching I baths. Addition of quantities in excess of 2.5 though having no apparent harmful effect does not appear to accelerate the aluminum dissolution further. The preferred range of concentration for the accelerating metal ion is' generally in the order of from 0.5 to 1% based on the weight of the alkali, the 1% upper limit being dictated primarily .by the cost of the salt used.

In the practice of the present invention, an etch g bath containing 1 to 20% of an etching alkali is made I I up with a salt of cobalt in an amount suflicient to pro- Where such scale 3 vide 0.2 to 2.5% cobalt based on the caustic, and, if desired, 1 to 5% based on the alkali, of an antiscalant. The components of the bath may be mixed first, as for example in a dry mix that can be packaged and stored, which is later added to water in preparing the etching bath separately. The etching bath may be operated at any suitable temperature such as 120 to 212 F., and replenished with the necessary components when it becomes depleted through use.

In the preferred practice, where a very rapid etch is desired without going to the high alkali concentrations of co-pending application Serial No. 544,584 hereinabove identified a relatively strongly alkaline etching bath on the order of 8 to 15% caustic soda, is employed at relatively high temperatures, of about 180 to 200 F. The accelerating metal is added in amounts of from 0.5 to 1%, based on the weight of the alkali. From 1 to 5% of an antiscalant based on the alkali, may also be added, the preferred antiscalant being any of the group consisting of sorbitol, gluconate and the glucarnines. When the aluminum stock has been etched in this bath to the desired degree, it is removed, rinsed and then preferably desmutted in a 15 to 20% nitric acid solution.

It is generally preferred to provide all the etching ingredients in a single package in a dry form, the etching solution then being prepared by adding this to water. In preparing such a composition, etching alkali, preferably caustic soda, is mixed with the cobaltous salt in amounts such that the final composition will contain about 0.2 to 2.5 parts by weight cobaltous ion per 100 parts of caustic. In the preferred practice, sufficient scale inhibitor is included to give a composition containing about 0.2 to 2.5 parts by weight of cobaltous ion and 1 to 5 parts scale inhibitor per 100 parts of caustic.

An example of such a dry composition may contain, for each 100 parts of caustic soda, about 0.6 to 7.0 parts by weight cobaltous chloride dihydrate (corresponding to 0.2 to 2.5 parts cobaltous ion) and 1 to 5 parts by weight sorbitol. In preparing the etching bath the etching composition is then added to water to give a solution having the caustic concentration desired, which is generally in the order of 1 to 20%.

In preparing dry formulations it is sometimes advantageous to add small amounts of certain other ingredients such as pine oil or nitrates as de-dusting or brightening agents, etc.

In order to illustrate more clearly the benefit of the results obtained by the invention, the following example is given:

Example I An etching bath was made up to contain 4% caustic soda. At 150 F., this bath caused a panel composed of bath, precisely similar in all respects except that it also contained 2.8% cobaltous chloride dihydrate (that is, 1% cobaltous ion) based on the caustic, was used to etch a second panel and gave an etching rate of .0150 gram per minute per square inch of surface, an increase of 16%. After-treatment of the etched articles consisted in rinsing with water and desmutting by immersion in aqueous 15 nitric acid solution for a few seconds. The second panel (treated in the accelerated bath) was etched to a depth of 3.3 mils after 10 minutes, and the etched surface was uniformly smooth. The first panel was etched to a depth of only 3.0 mils after treatment for the same length of time in the unaccelerated bath.

Having described my invention, I claim:

1. The method of etching aluminum alloys containing at least 0.1% copper involving relatively rapid dissolution of substantial quantities of said aluminum in a hot aqueous solution of an etching alkali, said method comprising treating the surface of said aluminum alloys with an aqueous solution of an etching alkali containing cobaltous ion at a temperature of at least F., said cobaltous ion being present in an amount of at least 0.2% based on the alkali present.

2. The method of claim 1 wherein said solution also contains an antiscalant.

3. The method of claim 1 wherein said etching alkali is primarily caustic soda.

4. The method of etching aluminum alloys containing at least 0.1% copper involving relatively rapid dissolution of substantial quantities of said aluminum in a hot aqueous solution of an etching alkali, said method comprising treating the surface of said aluminum alloys with an aqueous solution containing 1 to 20% of an etching alkali, 1 to 5% based on the alkali of an antiscalant and at least 0.2% based on the alkali of cobaltous ion at a temperature of at least 120 F.

5. The method of etching aluminum alloys containing at least 0.1% copper involving relatively rapid dissolution of substantial quantities of said aluminum in a hot aqueous solution of an etching alkali, said method comprising treating the surface of said aluminum with an aqueous solution of caustic soda having a temperature of to 200 F., said solution containing 8 to 15% caustic soda, 1 to 5% based on the caustic soda of at least one antiscalant of the group consisting of sorbitol, glucamine and gluconate, and 0.5 to 1% based on the caustic soda of cobaltous ion.

References Cited in the file of this patent UNITED STATES PATENTS 2,168,909 Mason Aug. 8, 1939 2,650,875 Dvorkovitz et al Sept. 1, 1953 FOREIGN PATENTS 858,931 France May 20, 1940 

1. THE METHOD OF ETCHING ALUMINUM ALLOYS CONTAINING AT LEAST 0.1% COPPER INVOLVING RELATIVELY RAPID DISSOLUTION OF SUBSTANTIAL QUANTITIES OF SAID ALUMINUM IN A HOT AQUEOUS SOLUTION OF AN ETCHING ALKALI, SAID METHOD COMPRISING TREATING THE SURFACE OF SAID ALUMINUM ALLOYS WITH AN AQUEOUS SOLUTION OF AN ETCHING ALKALI CONTAINING COLBALTOUS ION AT A TEMPERATURE OF AT LEAST 120*F., SAID COBALTOUS ION BEING PRESENT IN AN AMOUNT OF AT LEAST 0.2% BASED ON THE ALKALI PRESENT. 